Excimer formation in zinc-phthalocyanine revealed using ultrafast electron diffraction

• 成果类型: Article
• 署名作者: Hammer, Sebastian; Britt, Tristan L.; Kremeyer, Laurenz; Roedel, Maximilian; Cai, David; Pflaum, Jens; Siwick, Bradley J.
• 署名单位: McGill University; McGill University; University of Wurzburg
• 刊物名称: PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
• ISSN/ISSBN: 0027-10619
• DOI: 10.1073/pnas.2411975121
• 发表日期: 2024-12-17
• 关键词: dimers

摘要

The formation of excited dimer states, so called excimers, is an important phenomenon in many organic molecular semiconductor solid state aggregates. In contrast to Frenkel exciton-polarons, an excimer is long-lived and energetically low-lying due to stabilization resulting from a substantial reorganization of the intermolecular geometry. Here, we show that ultrafast electron diffraction can follow the dynamics of solidstate excimer formation in polycrystalline thin films of a molecular semiconductor, revealing both the key reaction modes and the eventual structure of the emitting state. We study the prototypical organic semiconductor zinc-phthalocyanine (ZnPc) in its crystallographic a-phase as a model excimeric system. We show that the excimer forms in a two-step process starting with a fast dimerization (approx. 0.4 ps) followed by a subsequent slow shear-twist motion (14 ps) leading to an alignment of the it-systems of the involved monomers. This structural distortion persists well beyond 300 ps. Furthermore, we show that while the same excimer geometry is present in partially fluorinated derivatives of ZnPc, the formation kinematics slow down with increasing level of fluorination.